Process for rehabilitating internally reinforced concrete by electrical treatment

ABSTRACT

A process for rehabilitation of internally reinforced concrete which comprises applying to surface areas of the concrete a temporary sprayed-on layer of a self-adherent, electrolytic material comprised of cellulosic pulp fiber mixed with liquid electrolyte. A distributed electrode means, preferably a wire grid, is embedded between layers of the self-adherent coating, and preferably is supported by spaced-apart, non-conductive battens. Voltage sufficient to develop a current of about 1-5 amps/m 2  is applied between the reinforcement and the distributed electrode, to cause migration of chloride ions from the concrete into the electrolytic coating. When the chloride content of the concrete has been reduced to a desired level, the voltage is discontinued and the adherent electrolytic coating and distributed electrode are removed. Preferred electrolytes include water or other solutions, such as calcium hydroxide. Desirably, the distributed electrode is formed of a ferrous material reactive with chlorine, to minimize the release of free chlorine gas into the ambient.

RELATE APPLICATIONS

The present application is a continuation of my prior application Ser.No. 875,456, filed Apr. 29, 1992, now abandoned. Application Ser. No.875,456 was a division of my prior application Ser. No. 539,069, filedJun. 15, 1990, now U.S. Pat. No. 5,228,959, granted Jul. 20, 1993, whichin turn was a division of application Ser. No. 366,204, filed Jun. 9,1989, now U.S. Pat. No. 5,198,082, granted , Mar. 30, 1993, which inturn was a continuation-in-part of application Ser. No. 352,266, filedMay 16, 1989, now abandoned. Application Ser. No. 352,266 was acontinuation-in-part of application Ser. No. 100,834, filed Sep. 25,1987, now U.S. Pat. No. 4,832,803, granted May 23, 1989.

BACKGROUND AND SUMMARY OF INVENTION

The present invention is directed to a method for rehabilitatinginternally reinforced concrete by removal of chlorides.

One of the serious problems in connection with the maintenance ofreinforced concrete is the matter of corrosion of the internalreinforcement. In many cases corrosion of the reinforcement is caused bythe chloride contamination. This may result from the gradual absorptionof chlorides over a period of time or, in some cases, from theincorporation of chlorides into the original concrete to acceleratesetting. Conventional techniques for repairing chloride-contaminatedconcrete involve physical removal of the contaminated material andreplacement with fresh concrete. This is obviously a costly anddisruptive solution, at least with respect to vertical and overheadsurfaces.

It has been proposed heretofore to employ electrolytic techniques forremoving chlorides by ion migration. An article by J. E. Slater,Materials Performance, 1976, pp 21-26, describes such a method, whichinvolves applying an electric potential between internal reinforcementand a surface electrode submerged within a liquid electrolyte containedon the surface of the concrete. With the surface electrode forming thepositive pole of the electric field, the negative chloride ions withinthe concrete are caused to migrate through the concrete and out into theelectrolyte where they are oxidized to chlorine gas on the positiveelectrode or react chemically with components in the electrolyte.

The experiments described in the Slater article were carried out onchloride contaminated bridge decks, where the reinforcement hadcorroded. The bridge deck area was divided into sections of about 3.5 m²that were individually treated. These sections were provided with sealeddams for containing a liquid electrolyte solution. The electrolyte usedwas a calcium hydroxide solution, with and without ion exchangers.Slater used voltages of between 100 and 120 V, and the current variedbetween 28 and 100 amps per section. Slater was able to remove up to 90%of the chlorides in the concrete within a 24 hour period. Where theelectrolyte was used without ion exchangers, chlorine gas developed onthe platinized titanium electrodes and was released as free chlorinegas.

The Slater technique has not been commercially successful, for severalreasons: For one, the safety considerations are substantial whenoperating at voltages as suggested by Slater. More importantly, theSlater procedure is useful only for removing chlorides from the uppersurface of a horizontal slab. However, the more conventional method ofremoving the concrete is relatively simple and inexpensive when dealingwith an upwardly facing horizontal slab. The Slater method may well bemore costly than conventional techniques.

The present invention provides an economical electrolytic method ofremoving chlorides from contaminated reinforced concrete that can becarried out safely and with reasonable energy requirements and which,importantly, can be utilized conveniently and economically on verticaland downwardly facing surfaces. In U.S. Pat. Nos. 4,832,803 and5,198,082 (Ser. No. 366,204), mentioned above, it is proposed,generally, to employ removable, cohesive, self-adherent materials aselectrolytic media in electrolytic chloride removal procedures. Adistributed electrode means is embedded within the adherent coating andforms the positive terminal of the electrical system. When the processhas been completed, that is, when the level of chloride contaminationhas been reduced to a desired level, both the adherent coating and theelectrode means are removed from the face of the concrete. In thisrespect the process of the invention differs markedly from cathodicprotection systems, for example, where systems are permanently installedfor continuously maintaining an electric potential between internalreinforcement and surface electrode means.

In the above-mentioned U.S. Pat. No. 4,832,803, a material such asretarded gunite is disclosed as one of the materials suitable for use asan adherent electrolyte coating having adequate conductivity and beingremovable upon completion of the procedure. In an especiallyadvantageous and still further improved procedure according to thepresent invention, the adherent electrolytic coating material is formedof a mixture of cellulosic pulp fiber and water or other solution, whichis self-adherent to the surface of the concrete. The pulp fiber, whichis advantageously derived from re-cycled newsprint, is mixed with theliquid solution at the outlet of a spray nozzle, and the fiber-liquidpre-mix is sprayed onto the surface of area to be treated. The surfaceof the concrete draws some of the solution from the sprayed mixture andcauses the mixture to adhere tenaciously to the concrete surface.

Pursuant to the invention, the concrete area to be treated is tested bythe taking of cores and testing for chloride content. From these initialtests, the approximate time required to achieve a desired level ofchloride reduction can be estimated. The treatment can be continueduntil approximately the estimated time has passed, after which a furtherset of cores may be taken to establish the final processing conditions.

For a more complete understanding of the features and advantages of theinvention, reference should be made to the following description ofpreferred embodiments and to the accompanying drawing.

DESCRIPTION OF THE DRAWING

FIG. 1 is a fragmentary elevational view of a concrete wall beingprepared for treatment according to the invention, with parts brokenaway to show certain additional details.

FIG. 2 is an enlarged fragmentary cross sectional view as takengenerally on line 2--2 of FIG. 1.

FIG. 3 is a simplified representation of a typical voltage-versus-timecurve of a reference voltage monitored to determine the condition of thereinforcing steel of the concrete under treatment.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawing, the reference numeral 10 designates in ageneral way a body of a steel-reinforced concrete structure, which maybe in the form of a vertical wall or overhead structure. The concretewall typically includes a series of embedded steel reinforcing bars 11of conventional type.

Pursuant to the teachings of our parent application, a concretestructure 10, 11 that has become contaminated with chlorides, can berehabilitated by applying an adherent electrolytic coating 12 to onesurface of the structure, preferably that which has become mostcontaminated. A temporary distributed electrode 13, advantageously inthe form of a grid-like structure of conductive wire, is embedded withinthe electrolytic coating 12. A source 14 of direct current voltage isconnected between the internal reinforcing bars 11 and the temporaryelectrode structure 13. Over a period of time, an electrolytic migrationof chloride ions from the internal regions of the concrete wall,adjacent to the reinforcing bar 11 and into the electrolytic medium 12,is effected by the electric field. In accordance with broader aspects ofthe invention, after a predetermined period of treatment, wherein thechloride content of the concrete has been sufficiently reduced, thevoltage source 14 is disconnected, the electrode 13 and electrolyticmedium are removed, and, typically, the outer surface 15 is coated witha sealant layer (not shown) to inhibit future infusions withchloride-containing compositions.

Significantly, the external electrode means 13 is of a distributednature. Most advantageously, this can be realized by utilizing agrid-like structure consisting of a first set of wires 16 arranged atright angle to a second set of wires 17, with the wires being spotwelded or otherwise joined at the intersecting points. A particularlyadvantageous form of electrode structure 16 is comprised of a grid ofwire 16, 17, as illustrated, where the wires are approximately six mm indiameter and are constructed to form a grid with openings of about 10-15cm on a side. The specific form of electrode structure can be variedsignificantly, of course, as long as the electrode structure isdistributed relatively uniformly throughout the entire surface areabeing treated.

In many electrolytic systems for the treatment of concrete, such ascathodic protection systems, for example, or the system of the Slaterarticle heretofore mentioned, the external electrode system is formed ofa material such as platinized titanium, which is non-reactive to themigrating chloride ions. In appropriate circumstances, the distributedelectrode used in the process of the invention may be formed of similarmaterials. Such materials have a disadvantage, however, of releasingfree chlorine gas to the surrounding ambient, unless some kind of ionexchange material is utilized. As will be understood, the release ofchlorine gas can pose safety hazards, unless ventilation is adequate.The utilization of ion exchange materials, on the other hand, involvesadded expense.

In a preferred practice of the invention, the external electrode 13 isformed of steel. During operation of the process, the free chlorine ionsreact with the steel, causing the steel to corrode. This substantiallyminimizes any release of free chlorine gas. Over a period of time, thecorrosion products will reduce the cross section of the individualwires, and it typically will become necessary to increase voltagesomewhat to maintain desired current flow densities. In certain cases,where the chloride contamination problem is particularly severe, theformation of corrosion products may actually cause discontinuities inthe distributed electrode structure, in which case the electrode 13should be replaced.

In a particularly advantageous practice of the invention, the wire gridelectrode 13 is supported spaced slightly away from the front face 15 ofthe concrete structure, by means of battens 18, typically in the form ofwooden strips about 2cm on a side. As shown in FIG. 1, the battens 18may be attached to the face of the structure in the form of a large gridwhich may, for example, be about two meters on a side. The distributedelectrode structure 13 is then secured to the battens 18, by stapling orother suitable means, so that the external electrode is supported inappropriately spaced relation to the surface 15 of the concretestructure.

The electrolytic medium 12 may, in principle, be formed of anyself-adherent material capable of retaining moisture and, in suchcondition, having an adequate level of conductivity. As described in theparent application, this may include certain forms of grout, retarded soas to not set up during the course of the treatment. Moreadvantageously, however, the electrolytic medium comprises apapier-mache-like cellulosic material formed of a mixture of cellulosicpulp in water or other solution, such as calcium hydroxide solution. Theprocedure advantageously utilizes a conventional form of paper-makingpulp, preferably but not necessarily formed of a recycled paper. Thecellulosic pulp material is applied to the surface of the concretestructure by being mixed with the solution in a mixing nozzle 19 andbeing sprayed upon the surface of the concrete structure as the mixedmaterials emerge from the nozzle 19.

Desirably, the concrete structure is prepared for the application of thecellulosic medium by initially being tested for humidity level, andadjusted if necessary. This can be accomplished by forming a recess inthe concrete in a suitable location and then sealing off the outlet ofthe recess long enough for the relative humidity level of the air in therecess to stabilize. The relative humidity of this air is then measuredby conventional instrumentation. If the relative humidity level is 90%or less, it is desirable to spray the outer surface of the concretestructure with water to the point of saturation. After a short period ofsurface drying, the mixture of cellulose pulp and water or othersolution is sprayed over the surface of the moist concrete. Because ofthe porous character of the concrete, the cellulosic pulp-water orpulp-solution mixture adheres strongly to the surface 15, because of thetendency for the liquid of the pulp mixture to be drawn into the poresof the concrete.

Desirably, the pulp-solution mixture is comprised of chopped pulp mixeduniformly with the solution in ratios of from about 2.7-l to about 4.0-lof water or other solution per/kg of dry fiber. The two materials arejoined at mixing nozzle 19, which is supplied with the pulp fiberentrained in air through an inlet tube 22, where it is joined with theliquid component, supplied through a tube 23. The pulp-solution mixturequickly sets up in the form of a papier-mache-like material, which isself-adherent to the concrete and has a high degree of coherency withitself. Advantageously, a first layer of the papier-mache-like materialis sprayed onto the concrete surface to a depth approximately equal tothe thickness of the wood battens 18 (e.g., about 2 cm). After thisfirst layer has been applied, the distributed electrode grid 13 isattached to the exposed surfaces 24 of the battens, overlying thejust-applied layer of the papier-mache. Thereafter, additionalpulp-solution mixture is sprayed over the top of the distributed gridstructure, to provide a total layer of approximately 4-5 cm inthickness.

The papier-mache-like material forming the electrolytic medium 12,because of its contained moisture content, has a sufficiently highdegree of conductivity to enable the process to be carried outadvantageously. Thus, the voltage source 14 may be connected to thesystem as soon as the pulp-liquid material has been applied in themanner described. Of course, it is necessary to maintain a certain levelof moisture in the papier-mache-like medium 12, and this is accomplishedby spraying the surface of the electrolytic medium 12 as often asnecessary. Usually, twice per day is adequate.

Pursuant to the invention, the voltage 14 is maintained until thechloride level within the concrete structure has been reduced to anacceptable level. Normally, core samples are taken at selected pointsbefore commencement of the procedure and, from the chloride content asmeasured from these initial core samples, the approximate requiredduration of the rehabilitation procedure may be estimated. As thatestimated period approaches, a further set of core samples may be taken,if desired, in order to establish with a higher degree of accuracy theremaining treatment required to bring the chloride level of concretedown to a predetermined, satisfactory level.

In a typical case, the voltage applied by the voltage source 14 isadjusted as necessary to maintain a current flow density between theinternal and external electrode systems of approximately one-five amp/m²of concrete surface. Usually, however, voltages are in all eventsmaintained at 40 volts or below for safety considerations.

As set forth in my co-pending application Ser. No. 539,069, provisioncan be made for monitoring the condition of the reinforcing steel toavoid its polarization over time. Especially where the reinforcing steelis maintained under tension, for example, in certain types ofpost-tensioned or pre-tensioned concrete structures, it would notnormally be considered to utilize a procedure, such as describedhereinabove, for removal of chlorides, because of the danger of hydrogenembrittlement of the steel in tension. Thus, as the process continues,the reinforcing steel gradually becomes polarized. When the polarizationreaches a critical level, which may occur within a couple of weeks timein a typical process, evolution of hydrogen is favored, and steel whichis in tension may become subject to hydrogen embrittlement. Suchconditions obviously would be highly detrimental to a tensionedstructure.

Desirably the condition of the internal reinforcing steel is monitoredperiodically. When the polarization reaches a danger level, theprocedure can be discontinued long enough for the polarization todissipate or the current flow may be reversed for a short period of timein order to dissipate the polarization at an accelerated rate.

To advantage, monitoring of the polarization is accomplished by the useof a reference half cell 25, which is embedded in the concrete, closelyadjacent the reinforcing bar. When the voltage developed between thereinforcing bar and the reference half cell (hereinafter referred to asreference voltage) reaches a predetermined level, indicating a criticaldegree of polarization, the desired process modifications (e.g., voltageinterruption or voltage reversal) may be carried out. By way of example,where the reference half cell 25 is a copper-copper sulphate cell, avoltage of minus 1000 millivolts would reflect the approach of adangerous condition, at which the process should be temporarily stoppedor the current reversed for a short period. Where the reference halfcell 25 is a lead-lead oxide cell, a measurement of minus 500 millivoltswould reflect a danger level.

In order to measure accurately the reference voltage between thereinforcing bar 11 and the reference half cell 25, as by the voltagemeter V, it is necessary to interrupt the application of primarytreatment voltage from the external source 14 at periodic intervals, forexample every ten minutes or so. As reflected in FIG. 3, when theexternal voltage is interrupted, the reference voltage dissipates alonga curve 30, rapidly at first and then more slowly as it approaches alimit condition representing the true reference voltage. After aninterruption of five to ten seconds, the curve begins to flatten out andit becomes evident to the observer whether the reference voltage isgoing to reach a predetermined voltage level 31, the value of which is afunction of the composition of the reference half cell.

In the illustration of FIG. 3, three interruption cycles areillustrated. In the first, at the end of the interruption period, thereference voltage is at a level indicated by the reference numeral 32,which is comfortably above the predetermined danger level. Accordingly,the external voltage from the source 14 is reinitiated. In the secondillustrated interruption cycle, approximately ten minutes later, thereference voltage decay curve 33 approaches but still does not reach thedanger level 31, and the external voltage is reestablished for a furthercycle. At the end of the third illustrated cycle, the curve 34 of thevoltage decay is shown to pass below the line known to indicate adangerous condition of the internal steel. At this time, the externalvoltage would either be discontinued for a sufficient period of time toallow polarization of the steel to be dissipated, or the externalvoltage might be applied in the reverse direction for a short period oftime.

As will be readily appreciated, the procedure is readily subject towholly automatic control through simple microprocessor circuitry, forexample, designed to interrupt the external voltage on a predeterminedperiodical basis and to monitor the decay curve of the referencevoltage.

Where desired, of course, the half cell monitor may be utilized inconnection with any internally reinforced structure, whether or not theinternal reinforcement is under tension. However, the use of suchcontrols is considered highly significant in connection with tensionedreinforcement.

The procedure of the invention provides a particularly advantageous andefficacious method of removing excess chlorides from concrete structuresin a substantially non-invasive manner. The use of a self-adherent,removable material as an electrolytic surface medium enables anelectrolytic technique to be carried on vertical surfaces as well asdownwardly facing overhead surfaces. The self-adherent medium remainssufficiently moist to provide for adequate levels of conductivity, whileat the same time remaining adherent and cohesive during use and easilyremovable at the end of the procedure.

In accordance with the present invention and to particular and specialadvantage, the electrolytic medium is comprised of a papier-mache-likematerial formed of cellulosic pulp, which typically may be newspapergrade pulp comprised of chopped pulp fiber. The pulp fiber may bevirgin, but for cost purposes is more desirably a recycled newsprint.The pulp material is mixed in situ with the liquid and sprayed on aconcrete surface, prepared by being moistened to a level of at leastabout 90% relative humidity. The papier-mache-like material, applied insitu, has important advantages to the process, because of its highdegree of self-adherence to the concrete surface, which enables it to beeasily utilized on vertical and/or overhead surfaces. Likewise, thematerial is inherently light in weight, which also facilitates its useon vertical and overhead surfaces as will be readily appreciated.Application of the papier-mache-like electrolytic medium to the surfaceof the concrete is simple and inexpensive, being carried out by means ofa spray nozzle which serves to both mix and apply the material. Becauseof the inherent light-weight character of the papier-mache material, itis typically desirable, when using such material, to separately supportthe external electrode grid, and this typically is done by means ofbattens of wood or other relatively non-conductive material.

The papier-mache-like material used for the electrolytic medium is easyto maintain. It may be remoistened easily by simply sprayingperiodically with additional water or other solution. It is also verydurable, in that it can easily be kept in place for the duration of-thetreatment period required, typically two to eight weeks. Additionally,and of particular importance, the papier-mache-like material may bereadily cleaned off at the end of the treatment, using high pressuresprays, for example, Disposal of the used material is very simple andrelatively inexpensive.

For most purposes, the process of the invention advantageously utilizesan external grid electrode formed of steel, which is embedded in theadherent electrolytic medium. Utilizing a steel electrode grid, thechloride ions freed from the concrete and migrating into theelectrolytic medium cause the steel to corrode, thereby producingcorrosion products with the steel, rather than to emit free chlorinegas. For many situations, the release of substantial amounts of freechlorine gas could not be tolerated for safety reasons. The use of asteel electrode grid, while it causes the electrode to be consumed bythe corrosion products, is a superior solution to providing for ionexchange, for example. In most cases, the development of the corrosionproducts can be compensated for by increasing voltage levels (up to themaximum desired level of 40 volts). In particularly aggravated cases,the electrode grid may have to be replaced after a period of time,before the process has been completed. While the process of theinvention does not preclude the use of more conventional electrodematerials, such as platinized titanium, the use of steel electrodes ispreferred and advantageous for most circumstances.

In any of its various modifications, the process of the invention mayadvantageously utilize monitoring of the polarization of the internalsteel reinforcement as a significant control criterion. In this respect,because of a danger of hydrogen embrittlement, it would not have beenconsidered feasible, heretofore, to utilize a process such as thedescribed electrolytic procedure for chloride removal, in situationswhere the internal steel reinforcement was maintained under tension,particularly in the case of pre-tensioned or post-tensioned structures.In one of its aspects, the procedure of the invention, incorporatesmonitoring the condition of polarization through means such as the useof an embedded reference half cell. As polarization of the internalreinforcement increases, with continued practice of the process of theinvention, the polarization is periodically monitored. When it reaches alevel at which evolution of hydrogen gas is favored, and thereforehydrogen embrittlement of the steel in tension is likely to occur, theprocess is either terminated for a period of time or the polarity of theimpressed voltage is reversed for a short period, to effect dissipationof the condition of polarization. By this means, the process may besafely practiced in conjunction with tensioned structures.

It should be understood, of course, that the specific forms of theinvention herein illustrated and described are intended to berepresentative only, as certain changes may be made therein withoutdeparting from the clear teachings of the disclosure. Accordingly,reference should be made to the following appended claims in determiningthe full scope of the invention.

I claim:
 1. In the process for rehabilitating internally reinforcedconcrete, and which comprises(a) applying a removable adherent coatingto a surface area of said concrete, (b) embedding in said adherentcoating a distributed electrode structure, (c) impressing a DC voltagebetween said electrode structure and said internal reinforcement, and(d) thereafter removing and disposing of said adherent material, theimprovement characterized by (e) forming said adherent coating bypremixing a fibrous cellulosic pulp with liquid to form an electrolyticmaterial which is strongly self-adherent to moist concrete, (b) sprayingonto an exterior surface of said concrete a temporary coating of saidself-adherent fibrous pulp material to form a light-weight, cohesive,durable and re-wettable papier-mache coating, (e) adjusting said DCvoltage from time to time, as necessary, to maintain a current flow offrom about 1 amp/m² to about 5 amp/m² of concrete surface, and (f)re-moistening said self-adherent fibrous pulp material a plurality oftimes to maintain electroconductivity throughout said rehabilitatingprocess.
 2. A process according to claim 1 wherein,(a) spaced battens ofnon-conductive material are applied to said exterior surface, (b) afirst layer of said self-adherent pulp material is applied to saidexterior surface to form a first portion of said temporary coating, (c)thereafter said electrode structure is installed externally of saidbattens and of said first layer, and (d) thereafter a second layer ofsaid self-adherent pulp material is applied over said electrodestructure, to form a second portion of said temporary coating.
 3. Aprocess according to claim 1 wherein,(a) said pulp is formedsubstantially of recycled news-print.
 4. A process according to claim 1wherein,(a) said concrete, if wetted prior to spraying of said pulpmaterial, is allowed to become free of surface water prior to sprayingthereon of said self-adherent coating.
 5. A process according to claim 1wherein,(a) said concrete is contaminated with chlorides, (b) saidelectrode structure is formed of ferrous material to readily react withfree chlorine released during said process and thereby minimize therelease of chlorine gas.
 6. A process according to claim 5 wherein,(a)said electrode structure comprises a mesh material formed of wire in therange of about 4 mm to about 8 mm in diameter and having a gridstructure forming openings of from about 10 cm to about 15 cm on a side.7. A process according to claim 1 wherein,(a) said voltage is limited toa value not greater than about 40 volts, notwithstanding that thecurrent flow may fall below the desired range.
 8. A process according toclaim 1 wherein,(a) said pulp fiber is premixed with liquid inapproximate amounts of from about 2.7-4.0 liter of liquid per kg of dryfiber.